Adaptive ground clutter cancellation

ABSTRACT

The present invention refers to an airborne radar device ( 1 ) comprising at least two antennas ( 2, 3 ) and clutter suppressing means ( 4 ). The radar device is arranged, via the antennas ( 2, 3 ) to send out radar pulses focused in main lobes ( 5 ) and the antennas are arranged to receive reflecting pulses. The antennas ( 2, 3 ) are separated from each other vertically. The radar device ( 1 ) comprises means ( 6 ) for transforming the received radar pulses into complex video signals in the form sequences of range bins (R k ). The video signals are represented in a first channel (K 1 ) and a second channel (K 2 ).

TECHNICAL FIELD

The present invention relates to an airborne radar device comprising atleast two antennas and clutter-suppressing means for suppressing groundclutter. The radar device is arranged to send out radar pulses focusedin main lobes via the antennas. The antennas are arranged to receivereflected radar pulses. The antennas are separated from each othervertically. The radar device comprises means for transforming thereceived radar pulses into complex video signals in the form ofsequences of digital samples, so-called bins, which representradar-reflecting objects at different distances within the antennalobes.

PRIOR ART

To detect objects, targets, of different types, for example aircraft,boats, cars, or to determine the topography of a certain area of land,it is known to use radar. A radar transmits and receives electromagneticpulses within a solid angle which is determined by the design of theradar antenna system. A radar antenna is preferably designed to collectthe emitted electromagnetic energy within a main lobe which must be asnarrow as possible, the larger the antenna surface the narrower the mainlobe. The horizontal lobe width becomes less with increasing extent inthe horizontal direction of the antenna. This correspondingly applies tothe vertical lobe width.

By being suitably constructed, the radar antenna can thus concentrateand transmit or receive signals within a small solid angle, which iscalled the main lobe or sometimes simply the lobe. The main lobe coversa sector with a certain lobe width in the horizontal and verticaldirection. Due to its construction, the radar antenna can have a biggerlobe width in the horizontal direction than in the vertical directionand vice versa. Different lobe widths can occur but the width ispreferably only one or a few degrees. The larger the antenna the betterthe directional indication obtained for the target through a narrowermain lobe. Better sensitivity is also obtained through increased antennagain. The main lobe direction is changed through mechanical turning ofthe antenna or through electrical phase control of the radiatingelements in an electrically controlled antenna ESA (Elektriskt StyrdAntenn).

The range of sensitivity of the radar in the radial direction isdetermined by among other things the energy of the received reflectedpulse which, in turn, is proportional to the emitted pulse energy. Thehigher the pulse energy, the greater is the range in the radialdirection which can be covered. At different distances and for a givenreflection surface of the target, the received pulse energy is changedas the inverse of the distance to the fourth power. The received energydecreases with increasing target distance to be masked by the thermalnoise of the receiver at the limit of the receiver sensitivity of thesystem. The range of sensitivity is frequently called distance coverage.In the angular direction, the coverage is determined by the lobe widthaccording to the above.

When the electromagnetic pulses hit an object, it is said that theobject is illuminated by the radar pulses. When the object isilluminated, the electromagnetic pulses are normally reflected inseveral directions depending on the shape of the object. A fraction ofthe pulse energy is reflected back towards the radar and is received viathe receiver antennas which forwards the respective received signals tothe respective receivers. The receivers include means for converting theradio frequency analogue signals into sampled video signals. Theanalogue output signal is normally divided into two componentsdesignated I and Q. The component I stands for “in-phase” and thedesignation Q stands for “quadrature phase” and is treated like a timesequence of complex numbers I+jQ.

The sampling frequency is approximately as high as the bandwidth B ofthe radar pulse. To achieve a time resolution Δt=1/B, a number ofparameters are used. After a signal-adapted filtering in the radarreceiver, a radar pulse with short time duration or alternatively aradar pulse with longer time duration and suitable modulation to producethe bandwidth B can be used for achieving the time resolution Δt=1/B.The time resolution Δt=1/B corresponds to a distance resolutionΔR=Δtc/2, where c is the speed of light. The sampling frequency is highenough to guarantee that all radar echoes will be sampled after thesignal-adapted filter. A sample t seconds after the emission of theradar pulse contains the radar echo from a target at the distance R=tc/2and within a lesser distance range of ΔR=Δtc/2=c/(2B). This distancerange goes by the name range bin or simply bin. The sampling time issynchronized with the pulse emission with the aid of a clock.

In different radar systems, different sets of antennas are used. In abistatic radar, the antennas are separated whereas in a monostaticradar, they are collocated. A monostatic radar is the normalconstruction in moving radar systems. Most commonly, the same antennasystem is used for both transmitting and receiving. In certain systems,several part-antennas are also used for receiving; this being the casein the present description of the invention.

In the case of a moving unit which comprises a radar device, thereceiver antenna is expediently placed next to or in the vicinity of theantenna which sent the pulses, for practical reasons for one thing, whenthe radar unit e.g. is airborne. The reason for this is that the mutualdistance between the antenna which sent the pulses and the antenna whichreceived the reflected pulses is known independently of the arrangementof movement of the moving unit.

If radar is used for finding or following certain specific targets inspecific environments/surroundings, it is known to use a number ofdifferent techniques depending on what target one is looking for. Thisis due to the fact that different surfaces reflect in different ways.Moreover, certain conditions must be taken into consideration if it isattempted to detect targets on the ground, other conditions if it isattempted to detect targets at sea and yet other conditions if itattempted to detect targets in the air. It is also decisive if it isattempted to detect moving targets or stationary targets.

All cases where radar is used have in common that the receiver producesthermal noise which tends to mask weak received signals. Thermal noiseis suppressed in known ways by matched filtering. Further suppressionand thus increased range coverage is obtained by increased pulse energyas above and also by suitably combining several pulse responses withrepeated transmission bin by bin.

Other undesirable information is so-called clutter, which is unwantedradar reflections. Such clutter can be surface clutter in the form ofunwanted ground reflections, so-called ground clutter, or in the form ofunwanted reflections at sea, so-called sea clutter. Clutter can alsoconsist of volume clutter due to rain, or of point clutter from largestructures such as, e.g. steel bridges. Regardless of the type ofclutter which is being considered, it is always desirable to suppressclutter and noise so that the reflections from the sought-after objectcan be distinguished more easily from clutter and noise. Depending onthe type of clutter which is referred to and depending on which type oftarget is referred to, a plurality of known techniques for suppressingclutter is applied.

A target mostly has less reflection surface than the background, e.g.the ground, which is why clutter signals contain more energy than thereflected pulses from the target. If too little energy is left in thereceived pulses, it can, therefore, be difficult or impossible todistinguish target from noise and clutter.

When the intention is to detect moving targets which are located incontrast against the ground, e.g. cars or aircraft, it is known to usevarious different techniques. For example, DPCA (Displaced Phase CentreAntenna) can be mentioned, which is a method for suppressing groundclutter in airborne radar. For DPCA, two antennas are used in thehorizontal plane and at least two pulses. By selecting the PRF, i.e.pulse repetition frequency, in a suitable way in relation to thedistance between the antennas and the speed of the unit which carriesthe radar device, it is possible to compensate for one's own speed,which ideally results in a ground clutter without the spectral widening,which otherwise occurs due to the movement of the radar relative to thereflecting ground.

A problem with DPCA is that the radar device is limited to horizontallymounted antennas. Another problem with DPCA is that it requires at leasttwo coherent pulses at the same radio frequency. To be tied to at leasttwo coherent pulses implies, among other things, that the system becomessomewhat delayed since the system constantly has to wait for the nextpulse to be able to carry out clutter suppression. Moreover, PRF must bematched precisely to the current speed of the unit which carries theradar device, something which is difficult to carry out and which, whenit fails, results in deterioration in performance.

Another example is STAP (Space-Time Adaptive Processing), which is amethod for clutter suppression for airborne radar and especially for AEW(Airborne Early Warning) radar. STAP utilizes both space and timecorrelations in the clutter to suppress ground clutter via atwo-dimensional filter for space and time. STAP can thus be seen as ageneralized DPCA with the above-mentioned problems.

In U.S. Pat. No. 559,516, an airborne moving radar device for detectionand measurements of angles of slow targets in the main lobe throughsuppression of clutter is shown. The device comprises three radarantennas (left, centre, right) for receiving a radar signal, theantennas preferably being placed beside one another, i.e. in thehorizontal direction. The three antennas cooperate in forming a mainlobe when there is pulse alteration. Each antenna is coupled to areceiver arrangement for converting an analogue signal into a digitalsignal. The receiver arrangements are coupled to a signal processingarrangement which converts the signals into a video signal. The signalprocessing arrangement also comprises means as described above fordividing the signals into I and Q components.

The signal processing arrangement comprises a clock arrangement whichcontrols the time of emission of pulses and the times of receiving thereflected pulses at the three receiver antennas. The arrangement makesuse of the familiar Doppler effect, i.e. the frequency shift of thereceived signal which primarily manifests itself as a systematic,linearly increasing additional phase displacement for each new pulse inthe sequence of radar pulses. Since the radar device is moving withrespect to both the ground and any targets, the Doppler shift isproduced due to the movement of the radar device relative to thereflecting objects. The latter Doppler phenomenon is suppressed at thethree antennas by known techniques for motion compensation.

The signal processing arrangement also comprises means for storinginformation from the three antennas at different times, which are moreclosely determined at times which are determined by the pulse repetitionfrequency. The stored information is used for comparing video signalsfrom the right antenna with video signals from the centre antenna andvideo signals from the left antenna with video signals from the centreantenna, and further means for the comparison between the resultingdifference signals. To suppress ground clutter according to U.S. Pat.No. 559,516, the said stored information and said signals are usedtogether with a Fourier analysis in the signal processing arrangement toenable phase corrections for clutter in the frequency domain to beperformed.

One problem with a radar device according to U.S. Pat. No. 559,516 isthat the device depends on a multiplicity of pulse emissions followingone another, i.e. on PRF which means that it is based on various timeconstants and also frequencies for each pulse emission which leads topoor flexibility of the device. U.S. Pat. No. 559,516 also provides aradar device for the detection of targets on the ground, which is verydifferent from a radar device intended for the detection of air targets.

All devices and methods for suppressing clutter described above make useof a multiplicity of coherent pulses following one another, with theabovementioned problems. The methods require that the same radiofrequency is used for achieving coherence among all the pulses whichwill be processed together. During the relatively long time periodrequired, countermeasure equipment can calibrate the radio frequency ofthe radar and emit interference at the current frequency and thus foilthe operation of the radar. There is thus a requirement for an improvedradar device where clutter is suppressed, preferably independently ofcoherent pulses and frequencies.

DESCRIPTION OF THE INVENTION

The invention intends to solve the above problems in previously knownradar technology for suppressing ground clutter.

The problem is solved with an arrangement and a method which provides anairborne radar device comprising at least two antennas andclutter-suppressing means for suppressing unwanted radar pulsereflections such as ground clutter. The radar device is arranged to sendout radar pulses focused in main lobes via the antennas. The twoantennas are arranged to receive reflected radar pulses. The antennasare placed with their centres separated in the vertical direction andboth with lobes pointing in the same direction as the main lobe. Theradar device comprises means for converting the received radar pulsesinto complex video signals in the form of sequences of digital samples,so-called bins, which represent radar-reflecting objects at differentdistances in the current lobe direction. The radar device thus comprisesmeans for converting the analogue signals into complex digital videosignals in the form of a number of bins which together represent themain lobe. The clutter-suppressing means are arranged to represent thesignals in a first channel and in a second channel.

The clutter-suppressing means are arranged to form a weighted sum of thereceived signals through adaptive digital lobe formation. The weightsare thus adapted automatically with the aid of the received signals sothat the weighted sum corresponds to a resulting antenna lobe with highsuppression of ground clutter from each individual ground element withinthe antenna lobe. This improves the detection capability for flyingtargets at the same distance but above the same ground element. Inparticular, the arrangement makes use of the fact that the weights varyin a systematic way as a function of the distance to the respectivereflecting ground element.

The invention is characterized in that the clutter-suppressing means isarranged in such a way that the clutter component ec for a certain binRk in the first channel is also found in the second channel multipliedby a complex constant C(R_(k)). The complex constant C(R_(k)) is thequotient between the complex antenna gain of the second channel and ofthe first channel in the direction of the ground for the current binR_(k). According to the invention, the complex constant is automaticallyadjusted in such a way that the zero position, which is individual foreach bin, is directed towards the respective ground segment, therebysuppressing the clutter signal. This will be explained in detail furtherbelow.

Since the different antennas are placed above one another, a reflectedpulse from the same point on the ground or from the same point on atarget will travel a different distance and such a path difference isdescribed in the complex plane mainly by a phase displacement of thevideo signals, i.e. a phase displacement of the common vector I+jQ andalso a certain amplitude effect. The complex constant C(R_(k)) describesthe combined phase and amplitude effect.

The clutter-suppressing means is arranged for estimating a complexconstant Ĉ(R_(k)) which describes how the signals from the receiverantennas are weighted together separately for each bin R_(k) so that theclutter component e_(c) will be suppressed with the formation of aresultant video output signal (Ψ). The estimated constant Ĉ(R_(k)) isintended to suppress the clutter component e_(c) in the resultant videooutput signal Ψ by subtraction of the second channel from the firstchannel multiplied by the estimated constant Ĉ(R_(k)). Theclutter-suppressing means is arranged to produce, by suppressing theclutter component, a zero position in the resultant antenna pattern ofthe resultant video output signal Ψ in the direction of the ground inthe current range bin.

The signal e_(f) from the target is weaker than the signal from theground, i.e. the clutter component e_(c), which is why suppression ofthe clutter component enables the target signal of interest to stand outin an analysis of the reflected pulses from the radar emission.

For each bin R_(k), the constant Ĉ(R_(k)) can be estimated with the aidof surrounding bins, because they vary in a systematic manner as afunction of the distance due to the geometry in the current application.

The radar device preferably provides a method and an arrangement forsuppressing unwanted radar reflections from ground and sea, so-calledground or sea clutter. The radar system according to the invention isintended to be arranged on a moving unit such as an aircraft and isintended to preferably detect targets in the air. In contrast to ASLU(Adaptive Side Lobe Suppression), where the suppression is done for asingle discrete direction, the method involves the clutter beingsuppressed for each range bin even though direction varies betweendifferent bins. Another advantage with the present invention is that themethod works with a single pulse, i.e. that it is not necessary to havecoherence between several different pulses. The fact that the methodoperates on a single pulse means that clutter can be suppressed with theinformation provided by the reflections from the single pulse. Thisdiffers from what has been previously known, e.g. from U.S. Pat. No.559,516, STAP or DPCA, which have already been described above.

The fact that coherence between pulses is not necessary means e.g. thatthe frequency is changing from pulse to pulse. Such changing offrequency is particularly useful when illuminating, e.g. an enemyaircraft, since a change in frequency can prevent an enemy aircraft frombeing able to send out interference signals to prevent radar calibrationof the target. Frequency change is also advantageous by reducing themeasurement uncertainty for estimating the constant Ĉ_(k) which has thesame value for different radio frequencies and in time intervals whichcan include several transmit pulses.

As mentioned above, the invention advantageously works without coherencebetween the pulses but it must be mentioned that the invention is notlimited in this respect but also operates with coherence.

Placing the radar antennas in accordance with the present inventionprovides the possibility of suppressing clutter in the verticaldirection, which is advantageous in the case of air targets. To suppressclutter in the vertical direction is not possible with a radar devicewith a number of antennas placed in the horizontal direction accordingto the prior art.

Vertical direction here means a mainly perpendicular direction when themoving radar device is located on the ground. Since the radar unit isintended to be used with flying objects, the antenna may roll with theplane. A maximum rolling angle of the antennas of ±15° relative to theground plane provides continuous good characteristics, but rollingangles exceeding ±15° can entail a certain impairment of performancewith respect to clutter suppression.

To facilitate the understanding of how the sampling is coupled to theground segments, an imaginary side view can be taken where the radardevice in a coordinate system is located on the y axis (correspondinghere to the vertical) at a certain distance from the point of origin.The radar device sends a ray (main lobe) diagonally down towards the xaxis (corresponding here to the ground plane), which main lobeilluminates a part of the x axis. The sampling of the signal correspondsto a number of ground segments within the main lobe. Seen from the side,these ground segments are located in different directions with differentangles from the radar device. The sum of all ground segments makes upthe part of the ground which is illuminated by the main lobe.Considering instead the whole process above, each ground segment willconsist of “distance rings” limited in the sideways direction by theextent of the main lobe.

Each such ground segment in the main lobe can be described as limited byradial lines of sight which emanate from a common point in the radardevice, which common point is common to all radial limiting lines forall segments located in the main lobe. The radial lines of sight thusextend from the radar device to the ground surface and divide up thepart of the ground surface which is illuminated by the main lobe in thesaid ground segment.

The term bin has already been described and relates to the segmentlimited by the sampling in the main lobe. In the description below, aspecific bin is given as R_(k), where the index k specifies the specificbin k (the kth bin) and R specifies the distance created by the sampledtime in the segment in the main lobe in the line of sight. The bin canthus make up the part of the line of sight which is projected on aspecific ground segment.

In summary, it can be said that the sampled signal in its discrete timeinterval can be described as the main lobe being divided up into anumber of discrete bits, segments/bins and that each discrete timeinterval corresponds to a certain ground segment coupled to the bin inthat the bin constitutes a projected image of the ground segment.

In the signal processing arrangement, the video signals are processed indifferent ways. As mentioned above, the signals are sampled signals fromthe different antennas and contain discrete time intervals which definea bin.

As mentioned earlier, signals received after each emitted pulse can betreated individually, which eliminates the need for a plurality ofpulses.

Below, a description of the method according to the invention follows,where x₁ is represented in the first channel and where x₂ is representedin the second channel.

The signals for range bin R_(k), and pulse or Doppler channel n, aredescribed as followsx ₁(R _(k) ,n)=a·e _(f)(R _(k) ,n)+e _(c)(R _(k) ,n)+e ₁(R _(k) ,n)  (1)x ₂ (R _(k) ,n)=b·e _(f)(R _(k) ,n)+C(R _(k))·e _(c)(R _(k) ,n)+e ₂ (R_(k) ,n)  (2)where a and b are complex constants, e_(f) is the target signal, e_(c)is a clutter signal and e₁ and, respectively e₂ are two mutuallyindependent stochastic processes, one for each channel. Furthermore,C(R_(k)) is a complex parameter which is determined by the geometry,i.e. the geometric relationship between the antennas included and therange bin R_(k).

The resultant clutter-suppressed video signal is thus formed as follows:$\begin{matrix}\begin{matrix}{{\Psi\left( {R_{k},n} \right)} = {{{\hat{C}\left( R_{k} \right)} \cdot {x_{1}\left( {R_{k},n} \right)}} - {x_{2}\left( {R_{k},n} \right)}}} \\{= {{\left( {{\hat{C}{\left( R_{k} \right) \cdot a}} - b} \right){e_{f}\left( {R_{k},n} \right)}} +}} \\{{\left( {{\hat{C}\left( R_{k} \right)} - {C\left( R_{k} \right)}} \right){e_{c}\left( {R_{k},n} \right)}} +} \\{{{\hat{C}\left( R_{k} \right)}{e_{1}\left( {R_{k},n} \right)}} - {e_{2}\left( {R_{k},n} \right)}}\end{matrix} & (3)\end{matrix}$where the parameter C(R_(k)) has been estimated and the estimate isdesignated by Ĉ(R_(k)). The above equation shows that the clutter signale_(c)(R_(k), n), is suppressed completely when the estimate Ĉ(R_(k)) isequal to the complex parameter C(R_(k)).

The parameter C(R_(k)) is distance-dependent and the distance-dependencecan be modelled according to the following polynomial model$\begin{matrix}{{C\left( R_{k} \right)} = {{c_{0} + {c_{1} \cdot R_{k}} + \ldots + {c_{M} \cdot R_{k}^{m}}} = {\sum\limits_{m = 0}^{M}{c_{m} \cdot R_{k}^{m}}}}} & (4)\end{matrix}$

This polynomial can also be written in the following vector formC(R _(k))=φ^(T)(R _(k))θ  (5)whereφ^(T)(R _(k))=└1,R _(k) , R _(k) ² , . . . ,R _(k) ^(M)┘θ^(T) =└c _(o) , c ₁ , c ₂ , . . . , c _(M)┘  (6)where θ^(T) is a transpose. To estimate the coefficients in thepolynomial, the following criterion function is used $\begin{matrix}{{v\left( {\theta,k} \right)} = {\sum\limits_{j \neq k}{\sum\limits_{n = 1}^{N}{{{{\varphi^{T}\left( R_{j} \right)}{\theta \cdot {x_{1}\left( {R_{j},n} \right)}}} - {x_{2}\left( {R_{j},n} \right)}}}^{2}}}} & (7)\end{matrix}$

Note that for the summation over the bins, the information from bin kwill be left out, i.e. the criterion is calculated in an environment ofbin k. An estimation of the parameter vector C(R_(k)) is thus obtainedby minimizing the criterion, see e.g. L. Ljung, T. Söderström, “Theoryand Practice of Recursive Identification”, Prentice Hall Inc., EnglewoodCliffs, N.J., 1981. The solution is obtained by{circumflex over (θ)}=P ⁻¹ r  (8)where the matrix P and the vector r are calculated as follows$\begin{matrix}{P = {\sum\limits_{j \neq k}{{\varphi\left( R_{j} \right)}{\varphi^{T}\left( R_{j} \right)}{\sum\limits_{n = 1}^{N}{{x_{1}\left( {R_{j},n} \right)}{x_{1}^{*}\left( {R_{j},n} \right)}}}}}} & (9) \\{r = {\sum\limits_{j \neq k}{{\varphi\left( R_{j} \right)}{\sum\limits_{n = 1}^{N}{{x_{2}\left( {R_{j},n} \right)}{x_{1}^{*}\left( {R_{j},n} \right)}}}}}} & (10)\end{matrix}$

An estimate of the parameter C(R_(k)) is thus obtained byĈ(R _(k))=φ^(T)(R _(k))ˆ  (11)which is used for suppressing the clutter signal.

From the above it can be noted that for the model order M equal to zeroin the polynomial, the following degenerated polynomial is obtainedC(R _(k))=c ₀  (12)

This polynomial provides the following vector

φ=1  (13)

If the number of pulses N is chosen as 1, the estimate of the parameterC(R_(k)) degenerates to $\begin{matrix}{{\hat{C}\left( R_{k} \right)} = \frac{\sum\limits_{j \neq k}{{x_{2}\left( R_{j} \right)}{x_{1}^{*}\left( R_{j} \right)}}}{\sum\limits_{j \neq k}{{x_{1}\left( R_{j} \right)}{x_{1}^{*}\left( R_{j} \right)}}}} & (14)\end{matrix}$By applying this estimate of the parameter C(R_(k)), it can be seen thatthe result becomes equivalent to a so-called matched filter. This meansthat an estimate of the parameter C(R_(k)) according to the invention isa generalization of the so-called matched filter. A matched filter isknown in connection with radar and is described e.g. in Robey, D. R.Fuhrmann, E. J. Kelly, R. Nitzberg, “A CFAR Adaptive Matched FilterDetector”, IEEE Transactions on Aerospace and Electronic Systems, Vol.AES-28, No. 1, pp. 208-216, January 1992.

According to an embodiment of the invention, the radar device comprisesan antenna arrangement in the form of a monopulse antenna comprising twoadjacent antennas which are above one another. One monopulse antennacomprises a system of antennas which can cooperate in transmitting apulse and which cooperate in a known manner to receive a radar echo.

An arrangement comprising more than two antennas placed above oneanother is also possible within the scope of the invention. For example,it can be said that three antennas give rise to two degrees of freedom.One degree of freedom can be used for clutter suppression and the seconddegree of freedom can be used for more precisely calibrating the currentaltitude of the target. To simplify the description of the invention,two embodiments of the invention comprising two antennas are describedbelow.

According to one embodiment of the invention, the radar device comprisesmeans for representing the video signal from the first antenna in thefirst channel and means for representing the video signal from thesecond antenna in the second channel. In the present embodiment, cluttercomponents will thus occur in both channels according to the abovedescription. As mentioned, the differences in magnitude between theclutter components are of such a type, that it is possible, by means ofthe estimation according to the invention, to suppress the cluttercomponents in the resultant video signals.

According to another embodiment of the invention, the radar devicecomprises means for summing the signals from pairs of antennas includedin the radar system in the second channel and means for forming thedifference between the signals from pairs of antennas included in theradar system in the first channel. In this embodiment, the secondchannel will be called sum channel Σ and the first channel differencechannel Δ, below.

The signals from the two adjacent antennas are transformed linearly intothe sum signal Σ and the difference signal Δ. The signals x1 and x2 areexchangeable for any suitable linear combinations, e.g. sum and delta.

When the reflected pulses come back to the antennas, the analogue outputsignals are added to one another and subtracted from one anotheranalogously in a known manner before they are converted into videosignals. The added analogue output signals are converted to a videosignal in a sum channel Σ. The subtracted analogue output signals areconverted into a video signal in a difference channel Δ. The sum channelprovides the sum of the contributions of the different antennas for eachbin and the difference channel provides the difference between thecontributions of the two antennas for each bin. The sum channelrepresents a sum lobe in a resultant antenna pattern and the differencechannel represents a difference lobe in a resultant antenna pattern. Thepresent invention thus comprises a radar antenna with the possibility toinstantaneously form at least two lobes in the form of, for example, asum lobe and a difference lobe, in the vertical direction.

According to the present embodiment, the invention is characterized inthat the clutter-suppressing means is arranged in such a manner that theclutter component e_(c) for a certain bin R_(k) is found in the sumchannel Σ multiplied by a complex constant C(R_(k)), where the complexconstant C(R_(k)) is the quotient between the complex antenna gain ofthe sum channel and the difference channel in the direction of theground for the current bin R_(k). This clutter-suppressing means isarranged in such a manner that the clutter component e_(c) for a certainbin R_(k) is found in the difference channel Δ. The clutter-suppressingmeans is arranged to estimate a complex constant Ĉ(R_(k)), whichdescribes how the signals from the receiver antennas are weightedtogether separately for each bin R_(k) in forming the resultant videosignal Ψ. The estimated constant Ĉ(R_(k)) has the purpose of suppressingthe clutter component e_(c) in the resultant output video signal Ψ inthe sum channel Σ via subtraction of the difference channel Δ multipliedby the estimated constant Ĉ(R_(k)). The clutter-suppressing means isarranged to create, by suppressing the clutter component, a zeroposition in the resultant antenna pattern of the resultant output videosignal Ψ in the direction of the current bin.

As mentioned earlier, the complex constant C between the sum channel(the second channel) and the difference channel (the first channel) isconstant for each bin R_(k).

The clutter component e_(c) for certain bins R_(k) is found by the sumchannel Σ multiplied by the complex constant C, where the complexconstant C is the quotient between the complex antenna gain of the sumchannel and the difference channel in the direction of the ground forthe current bin R_(k). As mentioned above, the complex constant isautomatically adjusted in such a manner that the zero position isdirected individually for each single bin towards the respective groundsegment thereby suppressing the clutter signal.

As described above, the complex constant Ĉ(R_(k)) has been estimated bymeans of a polynomial of suitable degree “m” with suitable coefficients“c_(m)”. The polynomial describes variations over a number of bins,centred around the current bin. The polynomial can be estimated on thebasis of the magnitude of the bin and values from a suitable selectionof bins located in the lobe.

To determine the complex constants, the method of least squares and asuitable polynomial of degree two are preferably used. The polynomialcan certainly be of both higher and lower degree. When estimating thecomplex constants, it is preferably assumed that the energy in theground clutter is much greater than the energy in the reflected pulsefrom the target, which is why the energy in the reflected pulse from thetarget can be disregarded in the estimation of the coefficients. If theenergy in the reflected signal were to be much greater than the energyin the clutter, the problem with the clutter would be solved, since thetarget provides a strong and clear signal and clutter suppression wouldnot be necessary.

The estimation of C(R_(k)) according to equation 14 has a systematicvariation with distance R_(k).

In the resultant video output signal Ψ(R_(k)) according to equation 3,the clutter component is suppressed for the current bin R_(k). Cluttersuppression can be illustrated in an antenna pattern for the resultantvideo signal Ψ(R_(k)) (clutter suppression signal) through a resultantzero position. The present method creates an adaptive space filter wherethe resultant clutter-free signal Ψ(R_(k)) can be used for subsequentfiltering and/or detection. In addition, it can be used, according tothe invention, to directly form a detector according to GLRT or AMFD.

As mentioned above, it is possible, with the two antenna lobes, to“zero” out, i.e. suppress, the clutter in the direction of the groundfor the current bin R_(k). Also due to the fact that several antennasare placed above one another, several spatial degrees of freedom areproduced which can be utilized for further improving the cluttersuppression and/or for more precise calibration of the target for eachsingle pulse.

In another embodiment of the invention, the analogue output signals fromeach antenna are converted to video signals. Sum channels and differencechannels are then formed by the first video signal and the second videosignal being summed in a sum channel for each bin and by the first videosignal being subtracted from the second video signal in a differencechannel for each bin. The subtraction is intended for calculating thedifference between the first video signal and the second video signalfor each bin. After that, estimation of the complex constant/vector isperformed in the same way as described in the case described above.

It applies to all embodiments that the antennas are allowed a certainamount of roll. A maximum rolling angle of the antennas of ±15° relativeto the ground plane provides continuous good characteristics, butrolling angles exceeding ±15° can entail a certain impairment ofperformance with respect to the clutter suppression.

Where there are more than two antennas, the complex constant C will berepresented by a vector C comprising components which give arelationship between different antenna pairs in the direction of theground for a given bin R_(k). Correspondingly, the estimated constant Ĉwill be represented by a vector Ĉ.

In the case of more than two antennas, the antennas included in theradar are treated in pairs with one another. The summing and subtractionaccording to the above can be done either according to the analogue caseor according to the digital case described above.

DESCRIPTION OF THE FIGURES

An embodiment of the invention will be described by means of the figuresshown in the drawings.

FIG. 1 diagrammatically shows a radar device according to an embodimentof the invention comprising two antennas;

FIG. 2 diagrammatically shows both a vertical and a horizontal view of aradar device according to FIG. 1 which sends out a main lobe;

FIG. 3 diagrammatically shows a radar device according to FIG. 1 seenfrom the side, where the main lobe is described as divided into segmentsdue to sampling;

FIG. 4 diagrammatically shows an antenna pattern for a radar deviceaccording to FIG. 1 according to an embodiment of the invention;

FIG. 5 diagrammatically shows a block diagram of the process of cluttersuppression according to the embodiment in FIG. 4;

FIG. 6 diagrammatically shows an antenna pattern of difference lobe Aand sum lobe Σ for a certain bin R_(k) according to an embodiment of theinvention;

FIG. 7 diagrammatically shows a block diagram of the process of cluttersuppression according to the embodiment in FIG. 6; and

FIG. 8 diagrammatically shows both a vertical view and a horizontal viewof an antenna pattern of the resultant lobes after clutter suppressionaccording to the embodiment in FIG. 4 or 6.

PREFERRED EMBODIMENTS

FIG. 1 diagrammatically shows a radar device 1 according to anembodiment of the invention comprising two antennas 2, 3 placed aboveone another in the vertical direction. As mentioned in the descriptionabove, the radar device can comprise more antennas than two. In thefigure, the radar device is shown to be round but the radar device canhave other forms, e.g. oval, square or polygonal, the antennas includedin the radar device being placed above one another.

FIG. 2 diagrammatically shows both a vertical view and a horizontal viewof a radar device according to FIG. 1 which sends out a main lobe 5which illuminates a portion of ground 10 and a target 11.

The upper part of FIG. 2 shows the vertical view of the main lobe 5 anda bin R_(k) located in the line of sight 8 a from the radar device 1.The line of sight 8 a is the direction in which the radar device sendsout its main lobe, i.e. the direction in which the main proportion ofthe energy of the pulse sent out is directed. In the vertical view, thebin R_(k) corresponds to a distance from the radar device 1 to a certainpoint along the line of site 8 a. The distance (bin) depends on thechoice of the instant of sampling and the sampling time selected.

The lower part of FIG. 2 shows the horizontal view of the main lobe 5.In the horizontal view, the bin R_(k) is represented by a ground segment13 on the illuminated portion of ground 10. In FIG. 2, the groundsegment 13 is shown as a curved strip with a certain width 14. Theground segment 13 is a projection of the bin R_(k) on the portion ofground 10 which is why the width 14 of the ground segment 13 depends onthe sampling time and the angle β. The angle β is the angle between theline of site 8 a of the radar device and a tangent 8 b to the groundplane 10. The tangent 8 b to the ground plane is taken at the pointwhere the line of site 8 a intersects the ground plane 10. The samplingtime gives rise to a distance ΔR_(k) which, depending on the angle β, isprojected on the illuminated portion of ground 10 according toΔR_(k)/cos (β), which gives rise to the width 14 of the ground segment13.

In FIG. 2, a bin R_(k) is shown which includes the target 10 which islocated at a distance from the ground.

FIG. 3 diagrammatically shows a radar device according to FIG. 1 seenfrom the side, where the main lobe 5 is described as divided intosegments/bins due to sampling. FIG. 3 shows a number of bins (bins R_(k)and R_(k+1) are marked in the figure) with corresponding ground segments13 _(k) (ΔR_(k)) and 13 _(k+1) (ΔR_(k+1)) with a certain width 14. FIG.3 is intended to illustrate how the reflected pulses, due to sampling,can be interpreted as the main lobe 5 being divided into a number ofbins. FIG. 3 also shows the target 11 in bin R_(k).

FIG. 4 diagrammatically shows an antenna pattern for a radar deviceaccording to FIG. 1. FIG. 4 shows a first lobe which is marked by x₁which corresponds to the signal which is received by the first antenna2.

FIG. 4 also shows a second lobe which is marked by x₂, which correspondsto the signal which is received by the second antenna 3. FIG. 4 alsoshows a target 11 at a certain distance from the ground 10. Both lobesx₁ and x₂ are directed towards the ground 10 and the target 11. It isshown in the figure that the second lobe x₂ comes from a position whichis vertically higher than the first lobe x₁. This is due to the factthat the two receiving antennas 2, 3 are placed above one another.Otherwise, the two lobes have the same extent and the same appearance.

FIG. 5 diagrammatically shows a block diagram of the process for cluttersuppression according to the embodiment in FIG. 4. The figure shows aradar device 1 comprising two antennas 2, 3. The device also comprises areceiver arrangement 9 which processes the signals from the antennas 2,3. The receivers 9 can contain means for converting analogue signalsinto digital signals. The signals x₁ and x₂ are signals from the twoantennas 2, 3 and FIG. 5 indicates that the signals are represented by afirst channel K, and a second channel K₂.

A method for performing clutter suppression according to the presentinvention will be described below, using the designations specified inFIG. 5.

The signals for range bin R_(k), and pulse or Doppler channel n, aredescribed as followsx ₁(R _(k) ,n)=a·e _(f)(R _(k) ,n)+e _(c)(R _(k) ,n)+e ₁(R _(k) ,n)  (1)x ₂(R _(k) ,n)=b·e _(f)(R _(k) ,n)+C(R _(k))·e _(c)(R _(k) ,n)+e ₂(R_(k) ,n)  (2)where a and b are complex constants, e_(f) is the target signal, e_(c)is a clutter signal and e₁ and, respectively e₂ are two mutuallyindependent stochastic processes, one for each channel. Furthermore,C(R_(k)) is a complex parameter which is determined by the geometry,i.e. the geometric relationship between the antennas included and therange bin R_(k).

The resultant clutter-suppressed video signal is thus formed as follows:$\begin{matrix}\begin{matrix}{{\Psi\left( {R_{k},n} \right)} = {{{\hat{C}\left( R_{k} \right)} \cdot {x_{1}\left( {R_{k},n} \right)}} - {x_{2}\left( {R_{k},n} \right)}}} \\{= {{\left( {{\hat{C}{\left( R_{k} \right) \cdot a}} - b} \right){e_{f}\left( {R_{k},n} \right)}} +}} \\{{\left( {{\hat{C}\left( R_{k} \right)} - {C\left( R_{k} \right)}} \right){e_{c}\left( {R_{k},n} \right)}} +} \\{{{\hat{C}\left( R_{k} \right)}{e_{1}\left( {R_{k},n} \right)}} - {e_{2}\left( {R_{k},n} \right)}}\end{matrix} & (3)\end{matrix}$where the parameter C(R_(k)) has been estimated and the estimate isdesignated by Ĉ(R_(k)). The above equation shows that the clutter signale_(c)(R_(k), n), is suppressed completely when the estimate Ĉ(R_(k)) isequal to the complex parameter CCR_(k)).

The signals x1 and x2 are exchangeable for any suitable linearcombinations, e.g. sum and delta.

FIG. 6 diagrammatically shows an antenna pattern of a difference lobe Δand sum lobe Σ for a certain bin R_(k). FIG. 6 shows an embodiment ofthe invention where the signals x₁ and x₂ from the antennas are linearlycombined in such a manner that a difference channel Δ and a sum channelΣ are formed. When the antennas receive reflections from the emittedpulses, the pulses are converted into output signals in the radarsystem. The output signals are added together in a sum channel Σ andsubtracted in a difference channel Δ. The difference lobes Δ are arepresentation of the difference channel Δ and the sum lobe Σ is arepresentation of the sum channel Σ.

FIG. 6 shows the target 11 in the sum lobe Σ. According to embodimentsof the invention, the clutter component ec for a certain bin R_(k) islocated in the sum channel Σ multiplied by the complex constant C, wherethe complex constant C is the quotient between the complex antenna gainof the sum channel and of the difference channel in the direction of theground for the current bin R_(k). In the sum channel Σ, the target isalso represented by a target signal e_(f).

FIG. 6 shows that the target 11 is located between the two differencelobes Δ, which is why the target 11 will not have a target signal in thedifference channel Δ. The figure shows that the lower one of the twodifference lobes “illuminates” a ground section, i.e. provides acontribution in the form of ground clutter e_(c) in the differencechannel Δ. This signal is used for suppressing the clutter componentCe_(c) in the sum channel, which also works for target positions whichgive rise to a target signal in the difference channel.

FIG. 7 diagrammatically shows means 4 for clutter suppression of theradar device 1 and a block diagram of the process in cluttersuppression, where the linear combination according to FIG. 6 is used.The radar device 1 sends out a pulse via the antennas 2, 3 in a timeinterval t₁, whereafter there is an interruption in the sending out in atime interval t₂. In time interval t₂, the receiver antennas 2, 3receive reflected signals from the pulse sent out.

The received pulses are converted into analogue output signals 7 in theclutter-suppressing means 4 via a radar receiver 6. The radar receivercan be any suitable means for converting received radar pulses intoanalogue output signals.

FIG. 7 also shows that the means 4 for clutter suppression comprisesmeans 15 for summing and/or subtracting the analogue output signals 7from the receiver antennas 2, 3 included in the radar system 1. Themeans 15 for summing and/or subtracting the analogue output signals 7can be formed by any suitable system for analogue signal processing.

The means 4 for clutter suppression also comprises means 18 forconverting the analogue output signals 7 into digital signals (A/Dconversion), video signals. In the embodiment shown in FIG. 5, A/Dconversion is performed after the analogue signals have been summed andsubtracted. During A/D conversion, the analogue signals are sampled. Asmentioned above, the sampling time is decisive for the appearance of thebins, i.e. how much information each bin can contain with respect toreflected pulses. The sampling time is thus decisive for the size of thebins and must be a fraction of the time interval t₂. The time intervalis so small, that the speed of the target and the radar arrangement,respectively, is negligible with respect to the problem set, i.e. thatthe problem can be considered as being static for a pulse.

The summed signals are presented in a sum channel Σ and the subtractedsignals are presented in a difference channel Δ. When comparing theembodiment described in FIG. 5 and the embodiment in FIG. 7, it can beseen that the first channel K₁ in FIG. 7 corresponds to the differencechannel Δ and that the second channel K₂ corresponds to the sum channelΣ. FIG. 7 shows means 16 for estimating the complex constant Ĉ. Such ameans 16 for estimating the complex constant Ĉ can be any suitablearrangement with the capability of performing algebraic operations insignal processing, e.g. a computer or a circuit or the like. Such anarrangement must also have the capability of performing the algebraicoperations necessary for estimating the complex constant Ĉ.

In FIG. 7, it is shown with known symbols that the estimated complexconstant Ĉ is multiplied by the difference channel Δ, whereafter thedifference channel Δ multiplied by the estimated complex constant Ĉ issubtracted from the sum channel Σ, which gives rise to the resultantvideo output signal Ψ. The algebraic operation is performed as abovewith an arrangement suitable for the purpose.

The factor which distinguishes the clutter component e_(c) in the sumchannel Σ from the clutter component e_(c) in the distance channel Δ isthus the complex constant C. As mentioned, the complex constant C isconstant for a given bin R_(k), which is why it is possible to estimatea complex constant Ĉ for each bin for each pulse. If the estimatedcomplex constant Ĉ is equal to the complex constant C, the cluttercomponent e_(c) in the sum channel Σ can be eliminated by multiplyingthe estimated complex constant Ĉ by the difference channel Δ, whereafterthe difference channel Δ multiplied by the estimated complex constant Ĉis subtracted from the sum channel Σ.

Equations (1), (2) and (3) apply to the embodiment with the sum channelΣ and delta channel Δ, where x1 is exchanged for Δ and x2 is exchangedfor Σ.

FIG. 8 diagrammatically shows both a vertical view and a horizontal viewof an antenna pattern of the resultant lobe for the resultant videooutput signal Ψ after clutter suppression according to any of theembodiments shown in FIGS. 4 and 6. FIG. 8 clearly shows how the clutteris suppressed and that the antenna pattern illustrates the suppressionas a zero position 17 in the resultant antenna pattern for the resultantvideo output signal Ψ. In this way, the signal contribution of thetarget stands out in the resultant video output signal Ψ in a mannerwhich enables further analysis to be performed.

The invention is not limited to what has been shown in the aboveembodiment but can vary within the scope of the patent claims. Asmentioned above in the description of the invention, the radar devicecan comprise more than two antennas. Moreover in another embodiment ofthe invention according to FIG. 6, the A/D conversion can be performedbefore the summing and subtraction of the signals.

1. An airborne radar device comprising at least two antennas andclutter-suppressing means, the radar device being arranged to send out,via the antennas, radar pulses focused in main lobes and the antennasare arranged to receive reflected radar pulses, the antennas beingseparated from each other vertically, the radar device comprising meansfor transforming the received radar pulses into complex video signals inthe form of sequences of bins (Rk), the video signals being representedin a first channel (K₁) and a second channel (K₂), characterized in thatthe clutter-suppressing means is arranged in such a way that the cluttercomponent (e_(c)) of a certain bin (R_(k)) in the first channel (K₁) isalso found in the second channel (K₂) multiplied by a complex constant(C(R_(k))), where the complex constant (C(R_(k))) is the quotientbetween the complex antenna gain of the second channel (K₂) and of thefirst channel in the direction of the ground for the current bin(R_(k)), the clutter-suppressing means being arranged to estimate acomplex constant (Ĉ(R_(k))) which describes how the signals from thereceiver antennas are weighted together separately for each bin (R_(k))when the resultant video output signal (Ψ) is formed, the estimatedconstant (Ĉ(R_(k))) being intended to suppress the clutter component(e_(c)) in the resultant video output signal (Ψ) by subtraction of thesecond channel (K₂) from the first channel (K₁) multiplied by theestimated constant (Ĉ(R_(k))).
 2. A radar device according to claim 1,characterized in that the radar device comprises means for representingthe video signal from the first antenna in the first channel (K₁) andmeans for representing the video signal from the second antenna in thesecond channel (K₂).
 3. A radar device according to claim 1, furthercomprising means for summing the signals from pairs of antennas includedin the radar system in the second channel (K₂) and means for forming thedifference between the signals from pairs of antennas included in theradar system in the first channel (K₁).
 4. Radar device according toclaim 1, wherein the clutter-suppressing means is set up for estimatingthe complex constant (Ĉ(R_(k))) by utilizing the values from range binsin the vicinity of the current range bin (Ĉ(R_(k))).
 5. A radar deviceaccording to claim 1, wherein the clutter-suppressing means is set upfor estimating the complex constant (Ĉ(R_(k))) by adapting a polynomialof degree “m” with coefficients “c_(m)”, wherein the polynomialdescribes variations over a number of bins centered around the currentbin.
 6. A radar device according to claim 5, wherein theclutter-suppressing means is set up for determining the coefficients ofthe polynomial by means of the method of least squares.
 7. A radardevice according to claim 1, wherein in that the clutter-suppressingmeans is set up for suppressing clutter without coherence betweendifferent pulses sent out.
 8. A radar device according to claim 1,wherein the antennas are rolled by ±15° maximum relative to the groundplane.
 9. A method for suppressing ground clutters comprising: jointlysending out a focussed radar pulse in the form of a main lobe from atleast two antennas separated from each other vertically, receivingreflected radar pulses by the antennas, converting the received radarpulses into complex video signals in the form of a number of bins(R_(k)), the video signals being represented in a first channel (K₁) anda second channel (K₂), the method comprising: transmitting a cluttercomponent (e_(c)) multiplied by a complex constant (C(R_(k))) for acertain bin (R_(k)) in the second channel (K₂), where the complexconstant (C(R_(k))) is the quotient between the second channel (K₂) andthe complex antenna gain of the first channel (K₁) in the direction ofthe ground for the current bin (R_(k)), transmitting the cluttercomponent (e_(c)) for a certain bin (R_(k)) in the first channel (K₁),estimating a complex constant (Ĉ(R_(k))) by weighting together thesignals from the antennas separately for each bin (R_(k)) when forming aresultant video output signal (Ψ), multiplying the estimated constant(Ĉ(R_(k))) by the first channel (K₁), in the resultant video outputsignal (Ψ), subtracting the second channel (K₂) from the first channel(K₁) multiplied by the estimated constant (Ĉ(R_(k))), which gives riseto the clutter component (e_(c)) being suppressed in the resultant videooutput signal (Ψ).
 10. The method according to claim 9, wherein themethod represents the video signal from the first antenna in the firstchannel (K₁) and the video signal from the second antenna in the secondchannel (K₂).
 11. The method according to claim 9, further comprisingsumming of the signals from pairs of antennas included in the radarsystem in the second channel (K₂) and subtracting the signals fromantenna pairs included in the radar system in the first channel (K₁).12. The method according to claim 9, wherein the step of estimating theestimated constant (Ĉ(R_(k))) comprises the following steps: selecting apolynomial of degree M with a number of complex constants (c_(m)),estimating the complex constants (c_(m)) by the method of least squaresand the values from a number of bins in the main lobe, which polynomialhas the following appearance:${\hat{C}\left( R_{k} \right)} = {\sum\limits_{0}^{M}{c_{m}R_{k}^{m}}}$13. The method according to claim 9, wherein the method suppressesclutter independently of the coherence between the pulses.
 14. Themethod according to claim 9, further comprising sending out andreceiving of pulses from antennas which are rolled by ±15° maximumrelative to the ground plane.
 15. The method according to claim 9,further comprising sending out and receiving of pulses from a radardevice which is airborne.